1. Technical Field
The present invention relates to the configuration of an image formed by an imaging apparatus such as copying machine and printer and particularly to an image forming method effective for the formation of a color image using an electrophotographic method or the like and an image-forming apparatus using same.
2. Related Art
This type of color image-forming apparatus has heretofore employed the following imaging steps to form a color image taking an embodiment using electrophotographic process as an example.
In some detail, light reflected by an original when irradiated with light is subjected to color separation by a color scanner. The color data thus obtained are subjected to image processing and color correction by an image processor to obtain a plurality of color image signals which are each then modulated by a semiconductor laser or the like to generate laser beams. These laser beams are each applied to an image carrier made of an inorganic photoreceptor such as selenium and amorphous silicon or an organic photoreceptor comprising a phthalocyanine pigment, bisazo pigment or the like as a charge-generating layer by plural times to form a plurality of electrostatic latent images. These electrostatic latent images are then sequentially developed with charged Y (yellow), M (magenta), C (cyan) and K (black) color toners. The toner images thus developed are then separately or altogether transferred from the image carrier made of an inorganic or organic photoreceptor onto an image support such as paper on which they are then fixed by a fixing unit of heat pressing fixing type. In this manner, a color image is formed on the image support.
In the aforementioned case, the color toner comprises an inorganic particulate material such as particulate silicon oxide, titanium and aluminum oxide or organic particulate material such as particulate PMMA and PVDF having an average particle diameter of from about 5 nm to 100 nm attached to a particulate material having an average particle diameter of from 1 μm to 15 μm having a colorant dispersed in a thermoplastic resin such as polyester resin, styrene-acryl copolymer and styrene-butadiene copolymer.
Examples of the colorant to be dispersed in the thermoplastic resin include benzidine yellow, quinoline yellow and Hansa yellow as Y (yellow) colorant, Rhodamine B, rose Bengal and pigment red as M (magenta) colorant, phthalocyanine blue, aniline blue and pigment blue as C (cyan) colorant, and carbon black, aniline black and blend of color pigments as K (black) colorant.
As the image support there has been heretofore used ordinary paper mainly composed of pulp material, coated paper obtained by spreading a resin mixed with a white pigment or the like over ordinary paper, white film made of a resin such as polyester mixed with a white pigment or the like.
On the other hand, as the transferring step there has been known a process which comprises previously allowing the image support to be adsorbed to a transferring roll or transferring belt composed of a dielectric material or the like provided opposed to an image carrier, and then applying a bias to the transferring roll or providing a predetermined transferring member (e.g., transferring corotoron, biased transferring roll or biased transferring brush) on the back side of the transferring belt, whereby an electric field having a polarity opposite that of the charge of the toner is given to the back side of the transferring roll or transferring belt so that the toner images are sequentially electrostatically transferred onto the image support.
As the transferring step there has been known also a process which comprises giving an electric field having a polarity opposite that of the charge of the toners to the back side of a belt-shaped intermediate transferring material composed of a dielectric material or the like provided opposed to the image carrier using a predetermined primary transferring member (e.g., transferring corotoron, biased transferring roll or biased transferring brush) so that the toner images formed on the image carrier are separately transferred onto the intermediate transferring material to form a multi-layer toner image thereon, and then giving an electric field having a polarity opposite that of the charge of the toners to the back side of the image support using a predetermined secondary transferring member (e.g., transferring corotoron, biased transferring roll or biased transferring brush) so that the toner images thus superposed on each other are electrostatically transferred onto the image support at once.
Further, as the fixing step there has been known, e.g., a heat pressing fixing process which comprises passing an image support onto which toner images have been transferred through the gap between a pair of fixing rolls having a heat source such as incandescent lamp incorporated therein disposed in pressure contact with each other so that the toners are heat-melted and fixed on the image support or a cooling/peeling fixing process which comprises passing an image support onto which toner images have been transferred with a fixing belt superposed thereon through the gap between a pair of fixing rolls disposed opposed to each other with the fixing belt interposed therebetween, the fixing belt having a release layer such as silicone resin layer formed thereon, extending over a plurality of tension rolls and comprising a heat source such as incandescent lamp incorporated therein, so that the toner images are heat-pressed and fixed, and then separating the toner images from the fixing belt after cooling so that the toner images are fixed on the image support.
It has been known that the latter fixing process is suitable particularly for the formation of an image having a gloss as high as that of silver salt photographic prints. Further, when the latter fixing process is used in combination with the aforementioned image support having a thermoplastic resin layer provided thereon, a uniformly high gloss can be obtained regardless of image density.
When as the base to be incorporated in the image support having a thermoplastic resin layer provided thereon there is used a white PET film or coated paper, the resulting image quality is good, but the image support itself is expensive. On the other hand, when inexpensive ordinary paper is used as a base, a technical problem arises that a good image quality cannot be obtained.
Further, when the thermoplastic resin is mainly composed of an amorphous polyester resin such as polyester-based resin, polystyrene-based resin and acrylic resin, a technical problem arises that all the requirements for low temperature fixability, heat resistance and mechanical strength cannot be satisfied at the same time.
In other words, taking into account the reduction of consumption of energy in the formation of image, low temperature fixability is essential. In order to satisfy the low temperature fixability, it is an effective solution to reduce the molecular weight of the resin or lower the glass transition point of the resin.
On the other hand, when an image having a smooth surface as photographic print is stored in automobiles or warehouses or allowed to stand in high temperature atmosphere during the transportation at the bottom of ship with the surface of an image superposed on the back surface of another, the surface of two images superposed on each other or the surface of an image superposed on an album material, it is likely that blocking can occur at the contact site.
In this case, in order to improve durability at high temperatures, i.e., heat resistance, it is effective to raise the glass transition point or molecular weight of the resin itself.
Further, the enhancement of resistance of image to folding, i.e., mechanical strength, too, is an important assignment. In order to enhance the mechanical strength, it is an effective solution to raise the molecular weight of the resin.
Thus, the enhancement of mechanical strength and heat resistance and the improvement of low temperature fixability are opposing assignments. In particular, in order to form an image having a gloss as high as that of silver salt photograph, it is necessary that the fixing temperature be raised. Therefore, it is more difficult to satisfy all the three requirements.